In the search for high performance materials considerable interest has been focused upon carbon fibers. Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength, and high modulus.
Polymeric materials heretofore commonly have been selected as the matrix material in which the relatively delicate carbon fibers are incorporated to form high performance materials with the carbon fibers serving as a light weight fibrous reinforcement. Epoxy resins most frequently have been selected as the matrix material. Additionally, the use of polyimides as the matrix material has been proposed primarily because of the ability of the polyimides to withstand even higher temperatures during use.
In order to improve the handleability of the carbon fibers without undue fiber damage during the formation of composite articles it has been the common practice to apply a flexible size or finish to the surface of the same. See, for instance, the epoxy size disclosed in U.S. Pat. Nos. 3,914,504 and 3,957,716. The sized carbon fibers bearing the epoxy coating are commonly incorporated in an epoxy matrix resin and a rigid composite article is formed upon curing. Epoxy matrix resins are commonly cured at temperatures below approximately 450.degree. F. If higher temperatures are encountered, the physical properties of the resulting composite article tend to be reduced.
There has remained a need for a highly satisfactory size or finish for use with carbon fibers which is capable of withstanding, without decomposition or loss of the desired size qualities, the more severe temperature conditions encountered when a polyimide serves as the matrix resin. For instance, polyimide resins commonly require an extended curing temperature at 550.degree. to 600.degree. F. At such temperatures epoxy sizes can be expected to undergo some level of degradation which can undesirably influence the physical properties of the resulting composite article.
Polyimide sizes also have been proposed in the prior art, but while being capable of withstanding the temperature involved during the curing of a polyimide matrix resin, have nevertheless been deficient in providing the desired improvement in handleability. Accordingly, the handleability of carbon fibers bearing these sizes generally has been very poor. For instance, if one selects as a size composition a common polyimide precursor solution containing at least one aromatic diamine, and at least one aromatic tetracarboxylic acid, and/or at least one aromatic diacid diester, the various components tend to lack the requisite film-forming characteristics to yield a satisfactory size. For instance, the monomers present tend to be dissolved solids and a brittle non-flexible and non-uniform coating tends to be deposited upon the surface of the carbon fibers which lacks those qualities commonly sought in size. This type of size results in extremely high levels of fuzz because of inadequate fiber protection and renders the weaving of the same virtually impossible.
It is an object of the present invention to provide sized carbon fibers which may be readily handled without undue fuzz formation or damage to the same.
It is an object of the present invention to provide improved sized carbon fibers which are capable of withstanding high temperatures (e.g., above 500.degree. F.) without deleterious results.
It is an object of the present invention to provide improved sized carbon fibers which are particularly suited for use in the formation of composite structures wherein a polyimide serves as the matrix material.
It is an object of the present invention to provide improved sized carbon fibers which produce minimal fuzz upon handling.
It is an object of the present invention to provide improved sized carbon fibers which are flexible and capable of readily undergoing impregnation with a matrix material.
It is a further object of the present invention to provide an improved carbon fiber reinforced composite structure comprising a polyimide matrix and a polyimide size coating on the carbon fibers which does not result in any substantial diminution in the overall physical properties of the composite structure.
These and other objects, as well as the scope, nature, and utilization of the claimed invention will be apparent to those skilled in the art from the following detailed description and appended claims.